phosphate question

john b.

Junior Poster
May 21, 2005
Tom, I read a post of yours on another forum which you said " inhibition of plant growth ranges for po4 are over 5ppm", does this mean over 5 ppm of po4 will affect growth or other nutrient uptake rates? If so , is this even if all other nutients are in proper range and how much over 5ppm would cause issues? Thanks,

Tom Barr

Staff member
Jan 23, 2005
Re: phosphate question

That means many folks have gone to 5ppm.
We do not know if anything causes issues above that.
Why would anyone want to add more than that?
Even a bad dosing error would not get over that.

Every once in awhile, someone thinks the KH2PO4 is KN03.
So it does happen but a water change brings it right back down.
Otherwise you have to add so much it starts to effect the KH buffering system(maybe 7-20ppm).

We add plenty for the plants at 1-2ppm.
There is a level that causes issues, but it's very high and impractical to find out critically since it likely effects the CO2/buffering system rather than merely PO4 inhibition impacting uptake.

Tom Barr